Use of magnets to provide resilience

ABSTRACT

The clamp-unit is arranged in class-1 double-lever configuration. The levers carry a pair of jaws at one end, and magnets at the other end. The magnets are arranged in repulsive pairs, whereby the magnets urge the jaws together.

This disclosure relates to clamping devices, optionally of anoperated-by-hand character, in which a person's hand operates against aresilience to open the jaws of the clamp, and the resilience serves tohold the jaws together when the clamp is released. The new design isoptionally for use in the field of clamping such items as circuit-boardsfor finished-production testing.

Exemplary apparatuses will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a sectioned side elevation of a clamping apparatus, which usespermanent magnets as the source of the clamping force.

FIG. 2 is the same view as FIG. 1 but shows the clamping apparatus in anopened configuration.

FIG. 3 is a plan view of a modified version of one of the levers of theclamping apparatus of FIG. 1.

The apparatuses shown in the accompanying drawings and described beloware examples. The scope of patent protection sought is defined by theaccompanying claims, and not necessarily by specific features of theexemplary apparatuses.

The clamping apparatus is used for clamping a circuit board 20 duringtesting. A clamp-unit 23 includes electrical components 24, whichinteract with the circuit board 20 being tested. The unit 23 alsoincludes wiring 25 for connecting the electrical components 24 to a basestation (not shown) containing the testing instruments etc.

The clamp unit 23 includes an upper-jaw 26 and a lower-jaw 27. The jaws26,27 are components of respective levers, being an upper-lever 30 and alower-lever 31.

The levers 30,31 include respective pivot-bosses 32,33, which are linkedby a pivot-pin 29.

The clamp-unit 23 includes an upper-magnet 34 and a lower-magnet 35. Themagnets 34,35 are carried at the respective magnet-ends 36,37 of the twolevers 30,31. The magnets 34,35 are arranged with the respectivesame-poles of the magnets facing each other, in opposition, whereby themagnets repel each other.

The magnets 34,35 are carried in respective magnet-sockets 38,39 formedin the magnet-ends 36,37 of the levers 30,31. The magnets themselves arein the form of circular discs, and the magnet-sockets 38,39 arecomplementarily circular, and sized such that the magnets fit snugly butstill loosely within the sockets. The sockets locate the magnets, andprevent the magnets from slipping laterally (sideways) with respect tothe levers 30,31.

The magnets may be fixed, e.g glued, into the sockets. However, once inplace in the sockets, the magnets are prevented from falling out of thesockets by the magnetic forces, whereby further fixing means might notbe needed.

The material of the levers 30,31 is non-magnetic. In the example, thematerial is hard plastic.

The following text-book convention is used to describe the variousclasses of arrangement of levers and the forces thereon. In a class-1lever, the pivot lies between the applied-force and the load. In aclass-2 and a class-3 lever, the arrangement is that the pivot is at oneend of the lever, and both the applied-force and the load lie to thesame side of the pivot. In class-2, the load lies between theapplied-force and the pivot, and in class-3 the applied-force liesbetween the load and the pivot.

Class-1 double-levers can be arranged in a cross-pivot or scissorsconfiguration; alternatively, class-1 double-levers can be arranged in arocker-pivot or clothes-pin configuration. The lever arrangement asshown in FIGS. 1,2 is a class-1 double-lever clothes-pin arrangement.

The clamp unit 23, as shown, was intended to be operated manually by aperson, especially by an inspector performing the task of inspectingcircuit-boards. The forces exerted by the magnets, and the geometry ofthe clamp unit (including the lever ratio) are such that the inspectorcan press the magnet-ends 36,37 of the levers 30,31 together, by the useof one hand, and can thereby open the jaws 26,27. The other hand then isavailable to remove the just-tested circuit-board and insert a fresh onebetween the jaws. With a fresh circuit-board in place, the inspectorreleases the clamp, and the jaws close onto the circuit board with thepre-determined force as exerted by the magnets. (Alternatively, ofcourse, the designer might choose to automate the task of opening andclosing the jaws.)

The lower magnet-socket 39 is so formed as to leave a thin wall 40 ofthickness T, covering the lower pole of the disc of the lower-magnet 35.

The clamp unit 23 is placed on a tabletop 42, which, in the exampleshown, is a sheet of steel. It is often the case that the inspectorwishes to be able to slide the clamp-unit 23 (with or without theunder-test circuit-board 20 clamped between the jaws 26,27 thereof)around, on the tabletop 42, and yet the clamp-unit remains firmly heldto the tabletop. That is to say, it is desired that the clamp unit canbe moved around the tabletop upon the inspector touching or grasping theunit and exerting hand forces on the unit, in order to move the unit;and it is desired that, in the absence of such touching or grasping, themagnetic attraction is sufficient to hold the clamp unit stationary andfirmly with respect to the tabletop.

This condition is achieved in the apparatus as shown. The magnet resistsseparation of the clamp unit from the steel tabletop. The magnet itself,however, is kept from touching directly against the tabletop by reasonof the separation due to the thickness T of the plastic material 40.

The designer might make the thickness T greater, whereby the magneticforce holding the clamp unit to the table is reduced—or might make thethickness T smaller, which increases the magnetic force. In order topermit the device to remain stationary and stable on the tabletop whenthe inspector is not moving the device around, the designer shouldarrange for the thickness T of the thin wall 40 to be small enough thatthe lower pole of the lower-magnet 35 is close enough to the steel toexert a substantial magnetic attraction. The clamp-unit 23 is then heldfirmly against the tabletop 42 by this magnetic attraction.

On the other hand, it is desired that the inspector be able to move theclamp-unit around on the tabletop. The resistance to such lateralmovement is the friction that acts parallel to the tabletop, which isproportional to the magnitude of the magnetic attraction, and thedesigner should arrange for the thickness T to be thick enough that thisfriction can be easily overcome by the forces exerted by the inspector'shand. Thus, if a lateral frictional resistive force is required of sayten Newtons, the coefficient of friction of plastic against (dry) steelbeing approximately 0.15, an attractive magnetic force of about sixtyNewtons would be required, and the thickness T should be setaccordingly.

The clamp-unit 23 requires a minimum of one pair of magnets. Preferably,the two magnets that make up the pair are arranged to repel each other.As shown in FIGS. 1,2 the designer will usually find it convenient toprovide two pairs of magnets. As shown in FIG. 3, four pairs may beprovided. If the magnets are not glued in, it is possible for themagnets to be slipped into and out of their sockets by hand, whereby, atleast in the FIG. 3 apparatus, the inspector can easily increase ordecrease the overall magnetic forces. The designer can also adjust theclamping force by setting the lever ratio of the clamping unitappropriately.

The magnets should be of such material and of such size as will providethe desired level of force. In the illustrated apparatus, the (disc)magnets were twelve mm diameter and five mm thick. The magnets wererare-earth magnets, specifically of nickel-plated neodymium-iron-boron.Each opposed pair of magnets exerted a repel force, over a relativetravel vector of six mm, of fifty Newtons. The jaws and the magnets wereroughly equidistant from the pivot axis. The thickness T of the thinwall was 1.5 mm.

As shown, the magnets are permanent magnets, and this is preferred forsimplicity, convenience and long-term reliability. However,electro-magnets are not ruled out. Electro-magnets might be useful, forexample, in that they permit simple automation. In some cases, themagnets (permanent, or electro-) may be supplemented by coilsprings, orother types of resilient springs.

In an apparatus that uses magnets in repulsion, the magnets have atendency to sideslip. Compared with compression coilsprings, designersusually have more options when it comes to arranging for the two ends ofa coilspring to remain in alignment throughout the stroke of thecoilspring, than to arrange for two magnets, in repulsion, to remain inalignment throughout their stroke. The magnets have no inherentresistance at all to sideslip, whereas a coilspring does have its ownstructure.

In the illustrated device, the structure of the device as a whole lendsitself to keeping the magnets in proper alignment. Mounting the magnetsin robust levers, with robust pivots, that are safe against any mode ofmovement other than the defined pivoting movement, counters any tendencythe magnets might have to sideslip, relative to each other and to thelevers. The levers are of robust rigid construction, as is the pivotstructure, and together these ensure that the levers remain aligned witheach other, and constrained against all modes of movements anddeflections other than the defined pivoting movement.

Held thus, the magnets are safe against tipping, rocking, sideslipping,and all the other possible modes of movement. The magnets can move onlyin the one mode of movement, i.e towards and away from each other alongthe path as defined and controlled and guided, by the pivot structure.The magnets cannot move relatively in any other mode.

It is noted that the force desired to be exerted on the circuit-board,to secure the circuit-board firmly between the jaws, often is of more orless the same order as the force that can be readily obtained from twopairs of simple disc magnets, of such size as described, arranged inrepulsive pairs. Thus, the desired magnitude of forces at the jaws ismost easily provided when the lever ratio between the applied force(from the magnets) and the load (between the jaws) is roughlyone-to-one. Although a one-to-one ratio is not impossible in the class-2and class-3 arrangements, practically the class-1 arrangement, asdescribed, is most convenient.

It is generally not desirable to locate strong magnets close to acircuit-board. It is all too easy for electronic components to beaffected, if not actually damaged, by being in close proximity to strongmagnetic fields. Again, the class-1 lever arrangement ensures that the(strong) magnets are safely spaced away from the possibly-sensitivecomponents—being the components 24 as well as the components of thecircuit-board 20.

The wiring 25 by which the unit 23 is connected electrically to the basestation usually is required, as shown, to be arranged in flexiblepig-tail fashion. It would be all too easy, when the biassing force ofthe clamp is provided by a coilspring, for the wires to become snaggedor tangled up in the coils, with the resulting possibility of damage tothe wiring when the clamp is operated. Nothing like that happens whenthe force is provided by magnets in repulsion.

It is recognised that disc magnets, in the size and shape as described,are simple to use and effective in performance. It is preferred to use aplurality of opposed pairs, to achieve a given force magnitude, ratherthan just one pair of large magnets. With a magnet in the form of alarge disc, it can be difficult to ensure that the effective centre ofthe magnetic pole is in the geometric centre of the disc, and, with alarge disc, it does not take much, by way of mis-match or offset of theopposing or facing same-poles, for the magnitude of the repel force tobe compromised. In an extreme, two opposed large-diameter disc magnetsmight enter a state in which they might even repel each other over onesector but attract each other over another sector, which would bedetrimental in the present case. With smaller-diameter magnets, themanner in which the magnet forces are exerted is much less likely toenter such spurious areas.

From this standpoint, a magnet that has a diameter-to-length ratio ofmore than about 1:1 might start to exhibit some instability, in theabove regard. Above 2:1, the likelihood of instability and/or effectiveloss of force is so great that magnets with such ratios preferablyshould not be employed.

The use of magnets for biassing components apart, or together, is wellknown. The use of magnets in clamping apparatuses, for holdingelectronic components in position while undergoing testing, is alsoknown. In lines 25-26 of column 4 of U.S. Pat. No. 6,445,200 (Haseyama,2002), referring to FIG. 7D, which relates to such testing, appear thewords: “the pressing unit may include a magnetic spring comprising apair of magnets arranged so that the same poles of the magnets areopposite to each other.”

It may be noted that the function of the clamp-unit as described hereinis to provide a clamping force, of the type and under the conditions asdescribed. The action of a clamp may be contrasted with, for example,the action of a cushion. In the Haseyama reference, even though thedevice is referred to as a “magnetic spring”, the spring is not used tobias a pair of jaws together, where the resilience of the biassingspring has to be overcome to force the jaws apart. Rather, in Haseyama,the magnetic spring is used as a force-limiting cushion.

As mentioned, the preferred lever configuration is the class-1double-lever clothes-pin configuration, in which the magnets are set torepel each other. It might be considered that the class-1 double-levercross-pivot or scissors arrangement would be equivalent, if the magnetsnow were set to attract each other. However, that arrangement is notpreferred, in that it is considerably more difficult to open a pair ofjaws, against a resilient force, by pulling levers apart, than to openthe jaws by pushing levers together. In the clothes-pin configuration,the clamp force at the jaws is set by the spring, and is predictable. Atthe same time, the inspector's hand force is readily available, andeasily applied, to open the jaws.

1. An operable clamping device, characterised by combining the followingfeatures: the device includes two levers, being a lever-A and a lever-B;lever-A carries a jaw-A at or near jaw-end-A of lever-A, and carries apivot-boss-A; lever-B carries a jaw-B at or near jaw-end-B of lever-B,and carries a pivot-boss-B; a lever-pivot connects the pivot-boss-A tothe pivot-boss-B, and defines a pivot-axis, about which jaw-A and jaw-Bcan undergo relative pivoting movement; lever-A carries a magnet-A;lever-B carries a magnet-B; magnet-A and magnet-B are so mounted on therespective levers that poles of the magnets lie facing each other;magnet-A and magnet-B are so mounted on the respective pivoting leversthat magnet-A and magnet-B exert a magnetic force on each other, andthereby urge relative movement therebetween; and the levers are soarranged that pivoting movement of magnet-A relative to magnet-B isaccompanied by pivoting movement of their jaw-ends.
 2. As in claim 1,further characterised in that magnet-A and magnet-B are arranged withlike-poles facing, whereby the magnets repel each other.
 3. As in claim2, further characterised in that: the clamp-unit is so structured that,as magnet-A and magnet-B move apart, so jaw-A and jaw-B move together;whereby the repulsive magnetic force causes the jaws to be pressedtogether.
 4. As in claim 2, further characterised in that: lever-Acarries magnet-A at or near magnet-end-A of lever-A; pivot-boss-A islocated intermediate between jaw-end-A and magnet-end-A of lever-A;lever-B carries magnet-B at or near magnet-end-B of lever-B;pivot-boss-B is located intermediate between jaw-end-B and magnet-end-Bof lever-B;
 5. As in claim 4, further characterised in that: lever-A,lever-B, and the lever-pivot, are arranged in class-1, double-lever,clothes-pin configuration; whereby pivoting movement apart of themagnet-ends of the levers is accompanied by pivoting movement togetherof the jaw-ends of the levers.
 6. As in claim 1, further characterisedin that lever-A, lever-B, and the lever-pivot, are arranged in class-1,double-lever, clothes-pin configuration; whereby pivoting movement apartof the magnet-ends of the levers is accompanied by pivoting movementtogether of the jaw-ends of the levers; magnet-A and magnet-B arearranged to repel each other, and thus to urge the magnet-ends apart,and thus to urge jaw-A and jaw-B together.
 7. As in claim 4, furthercharacterised in that the jaw-end-A carries an electrical component, andcarries an electrical lead that extends away from jaw-end-A towardsmagnet-end-A and beyond the magnet-end-A.
 8. As in claim 4, furthercharacterised in that the structure of the apparatus is such that: thejaws can be opened by squeezing together the magnet-ends of the levers,by hand; and the jaws can be clamped together by releasing themagnet-ends.
 9. As in claim 4, further characterised in that: magnet-Acomprises two magnets, and magnet-B comprises two magnets; and the twomagnets of magnet-A and the two magnets of magnet-B are arranged inrespective repulsive pairs.
 10. As in claim 9, further characterised inthat the respective magnet-ends of the levers are provided withrespective magnet-sockets, the magnet-sockets being so sized and shapedas to receive the magnets, and to locate the magnets against lateralmovement relative to the levers.
 11. As in claim 10, furthercharacterised in that each of the magnets is a permanent magnet.
 12. Asin claim 9, further characterised in that each magnet is in the form ofa cylinder, having flat ends, and the poles of the magnet are located inthe flat ends.
 13. As in claim 4, further characterised in that therepulsive force between magnet-A and magnet-B is in the order of fiftyNewtons.